Ultrahigh molecular weight polyethylene (hereinafter referred to as "UHMWPE") is commonly used to make prosthetic joints such as artificial hip joints. Wear of acetabular cups of UHMWPE in artificial joints introduces many microscopic wear particles into the surrounding tissues. The reaction to these particles includes inflammation and deterioration of the tissues, particularly the bone to which the prosthesis is anchored. Eventually, the prosthesis becomes painfully loose and must be replaced. It is generally accepted by orthopaedic surgeons and biomaterials scientists that the reaction of tissue to wear debris is the chief cause of long-term failure of such prostheses.
The literature describes numerous attempts to improve the wear resistance of polyethylene (hereinafter referred to as "PE") in joint replacements. Grobbelaar et al. [J. Bone & Joint Surgery, 60-B(3): 370-374 (1978)] attempted to improve the cold-flow characteristics of "high-density" PE prostheses made of Hostalen RCH 1000 C, without sacrificing its low-frictional properties, through a process of radiation crosslinking. Grobbelaar et al crosslinked the PE using high penetration gamma radiation in the presence of crosslinking gases, including acetylene and chlorotrifluoroethylene, or in an inert nitrogen atmosphere. Due to the absorption of the crosslinking gasses, the surface was more crosslinked than the interior of the polyethylene. Nevertheless, because of the high penetration power of gamma radiation, the PE became crosslinked throughout.
To improve the wear resistance of a medical prosthetic device, Farrar, WO 95/21212, used plasma treatment to crosslink its wear surface. This wear surface comprises a plastic material such as UHMWPE. Crosslinking was assumed to have occurred based on the presence of Fourier transform infrared (FTIR) absorption bands at 2890 cm.sup.-1. Farrar claims his ATR (attenuated total reflection) data imply that he had achieved a penetration depth of 0.5 microns, but the degree of crosslinking is not disclosed.
Streicher, Beta-Gamma 1/89: 34-43, used high penetration gamma radiation or high penetration (i.e., 10 MeV) electron beam radiation to crosslink UHMWPE and HMWPE specimens throughout their entire thickness. Streicher annealed the gamma irradiated material in a nitrogen atmosphere in order to increase crosslinking and reduce oxidation during long-term storage. Streicher found that the wear of the materials was greater after the crosslinking by electron beam radiation.
Higgins et al [Transactions of the 42nd Ann. Mtg., Orthopaedic Res. Soc., Feb. 19-22, 1996, p. 485] attempted to stabilize UHMWPE against oxidation after high penetration gamma irradiation (which crosslinked their specimens through the entire thickness) by reducing the concentration of free radicals. They used the following post-irradiation treatments: (1) pressurizing in hydrogen at 15 psi for 2 hours, or (2) heating at 50.degree. C. for 182 hours. They compared the amount of free radicals remaining in the PE using electron spin resonance (ESR), but they did not assess the impact of these treatments on the mechanical or wear properties of the UHMWPE, nor on the oxidation resistance.